This invention relates to a process for preparing a chromium-containing catalyst for the polymerization of olefins by applying to an inert inorganic support a complex of a chromium compound with one or more organometallic compounds of a metal of group II or III of the periodic system in which hydrocarbyl groups having 1-20 carbon atoms are bonded to this metal. The support plus chromium complex will be referred to as a "catalyst component". The active catalyst is obtained by heating the catalyst component.
The invention also relates to the polymerization with such a catalyst of an alpha-olefin having 2-8 carbon atoms, in particular ethylene, optionally together with amounts not exceeding 15 mole % of one or more other alpha-olefins having 2-15 carbon atoms.
Similar processes are known from U.S. Pat. No. 4,146,695, or are described in the copending application Ser. No. 261,738, now U.S. Pat. No. 4,382,020, and Ser. No. 320,563. The general method for forming supported chromium catalysts, by way of background, will now be summarized in the following materials.
The chromium (III) compound is complexed with organometallic compounds of a metal from group II or III of the periodic table, such as beryllium, magnesium, boron, aluminium or gallium. These group II or III compounds contain hydrocarbyl groups which are, preferably, alkyl groups having 1-20 carbon atoms.
Suitable organometallic compounds are in particular the aluminum trialkyls and the magnesium dialkyls.
The alkyl groups in the magnesium dialkyls may contain 2 to 12 carbon atoms, preferably 4 to 8 carbon atoms. Suitable organomagnesium compounds are di-aliphatics such as diethyl, ethylbutyl, dipropyl, di-isopropyl, dibutyl, di-isobutyl, diamyl, dihexyl, dioctyl, didecyl, and didodecyl magnesium, and also dicycloalkyl magnesium compounds with identical or different cycloalkyl groups having 3-12 carbon atoms, preferably 5 or 6 carbon atoms. Also, the dialkyl magnesium may contain a mixture of an alkyl and a cycloalkyl group. Although alkyl or cycloalkyl magnesium compounds are preferred, magnesium aryls may also be used, particularly diphenyl magnesium, but also ditolyl and dixylyl magnesium. The diaryl magnesium compounds are insoluble or poorly souble in aliphatic hydrocarbons and, for that reason, are dissolved in aromatic hydrocarbons. The organomagnesium compounds may be prepared in a known manner (see e.g. Organometallic Compounds; Vol. 1; G.E. Coates; M. L. H. Green and K. Wade; Organo-metallverbindungen; F. Runge). In particular, use may be made of solutions of magnesium alkyls prepared according to the process described in Dutch Patent Specification No. 139,981.
Suitable aluminum compounds are aluminum trialkyls and organoaluminum compounds having the general formula (R.sub.4).sub.2 A1H, where R.sub.4 denotes an alkyl group having 1-10 carbon atoms. Aluminum compounds containing one or more groups derived from a diene, as known from, for instance, GB No. 1.332.493 and U.S. Pat. Nos. 3,149,136 and 3,180,837, , may also be employed.
The reaction of the chromium compound with an organomagnesium or organoaluminum compound or another metal from group II or III is carried out in an inert solvent. This inert solvent is preferably a hydrocarbon, particularly one or a mixture of the linear or branched aliphatic hydrocarbons, such as butane, pentane, hexane, heptane, octane, decane, or the branched isomers of these, or a low-boiling gasoline consisting mainly of hexanes and/or heptanes, or a higher-boiling gasoline. Additionally, higher linear or branched saturated aliphatic hydrocarbons or mixtures thereof can also be used as the solvent. Although the chromium (III) compounds dissolve more readily in aromatic hydrocarbon solvents than in aliphatic hydrocarbons and can be reacted therein with magnesium diaryls which are also soluble therein, it is generally desirable not to use aromatic hydrocarbon solvents if adequate yields from the reaction in aliphatic and/or cycloaliphatic solvents are available, particularly in view of the high cost of aromatic hydrocarbon solvents, and also because of possible dangers to health occasioned by aromatic solvents.
Most chromium compounds will be dispersed in the solvent because of their low solubility in aliphatic and/or cycloaliphatic hydrocarbons. Dissolution may be promoted by gentle heating, e.g. at temperatures of 40.degree.-100.degree. C. or, if Low-boiling solvents are used, at the boiling point of the solvent (whether or not under pressure). Because of their low solubility, the chromium compounds will only slightly color the hydrocarbon solvent. However, addition of the organomagnesium or organoaluminum compound will yield dark-colored solutions as it reacts with the chromium compound and the colored product goes into solution.
Very suitable chromium compounds are complexes of chromium with 1,3-diketo compounds such as chromium (III) acetyl acetonate, chromium salts of mineral acids, and chromium salts of saturated or unsaturated aliphatic carboxylic acids.
In addition to the chromium complex, compounds or complexes of one or more other transition metals can also be deposited on the support. The mole ratio of the chromium to the other transition metals can vary within wide limits. In general, the composition of transition metals used should be such that the atomic ratio of chromium to the total other transition metals is between 50:1 and 1:50, preferably between 20:1 and 1:20, and particularly between 10:1 and 1:10. Polymer properties are influenced both by the choice of other transition metals and by the chromium/other transition metal mole ratio.
The ratio of the chromium compounds plus any other transition metal compounds, to the Group II or III organometallic compounds, expressed as the total atomic ratio of the transition metals to the metal of group II or III, is chosen between 1:0.5 and 1:20, preferably between 1:1 and 1:6.
The solution of organochromium complex and other transition metal complex of Group II or III organaometallic compound is combined with an inert inorganic particulate support to form the catalyst component by slowly, and with stirring, adding the solution containing the complexes to the support suspended in the solvent. The complexes can be deposited on the support by evaporation of the solvent if the complexes do not completely adsorb directly from solution. It is easily determined whether the complexes have adsorbed onto the support by simply noting whether the solvent has lightened as the support has become colored.
The inert inorganic support may be an oxide, such as silica, alumina, mixed alumina-silica, or oxides of zirconium, thorium or magnesium. Preferred among these are silica, alumina and alumina-silica mixtures, with silica being preferred in particular. Silica is known as an adsorbent and can be used in many different forms. Particularly suitable are silica xerogels of large pore volume. If desired, the silica can include other components, such as fluorine and titanium, in any known manner. Alternatively, these compounds can also be added after the catalyst component has been formed, either during or following activation.
The support is dried, insofar as necessary, by heating in dry air or nitrogen before the transition metal compounds are deposited. Drying should be such that the support is free of physically bound water.
The amount of complex chromium compounds deposited on the support may vary within wide limits but will generally be in the range of 0.01-10% by wt. of the support as chromium. As discussed previously, compounds of other transition metals can also be deposited on the support, in the ratios to chromium mentioned above.
The catalyst components prepared as described above have been found to contain highly reactive hydrocarbyl groups which are liberated by heating during activation. This decomposition presents difficulties for recovering the catalyst component, because during the recovery procedure the contact with air causes a spontaneous rise in temperature of the catalyst due to the uncontrolled reaction of the hydrocarbyl groups with oxygen. Moreover, these reactive hydrocarbyl groups pose a potential danger when the catalyst component is activated, because during the activation procedure the catalyst component is heated in a non-reducing atmosphere at a temperature of 200.degree.-1200.degree. C. During the activation, the alkyl groups are split off at relatively low temperatures, forming alkenes and alkanes. This decomposition is a rapid process, so that the activation chamber which contains fluidization gas comes to contain considerable amounts of these hydrocarbons. The flash point can easily be approached or even exceeded, and ignition by a spark or an electrostatic charge can lead to an explosion.